预镀Kr层抑制纳米多孔玻璃中$^4$ He的超流性
Suppression of Superfluidity of $^4$He in a Nanoporous Glass by Preplating a Kr Layer
摘要：
纳米多孔介质中的氦原子作为一种具有无序性和约束性的玻色模型系统引起了人们的广泛兴趣。在这里，我们已经研究了如何通过预镀氪单层的多孔结构的变化影响的超流体性质的$^4$他吸附或限制在纳米多孔Gelsil玻璃，它具有三维互连网络的纳米孔的直径为5.8 nm。氮的吸附和脱附等温线表明，单层预镀Kr的有效孔径减小到4.7 nm，并从裸Gelsil样品的锐分布的孔径分布加宽约8倍。用扭转振子研究了Kr预镀前后的吸附膜态和加压液体态的超流性质。在薄膜状态下，预镀Kr使超流转变温度$T_{\mathrm c}$和超流密度均降低约10%.薄膜超流性的抑制是由于$^4$ He原子的量子局域化的随机性在衬底电位，这是由预镀-诱导的孔径分布的加宽。在加压液体状态下，超流密度$ρ_{\mathrm s}$被发现增加了10%的Kr预镀，而$T_{\mathrm c}$在所有压力下下降了2%。$ρ_{\mathrm s}$的意外增强可能表明受限$^4$ He存在未知的无序效应。
英文摘要：
Helium in nanoporous media has attracted much interest as a model Bose system with disorder and confinement. Here we have examined how a change in porous structure by preplating a monolayer of krypton affects the superfluid properties of $^4$He adsorbed or confined in a nanoporous Gelsil glass, which has a three-dimensional interconnected network of nanopores of 5.8 nm in diameter. Isotherms of adsorption and desorption of nitrogen show that monolayer preplating of Kr decreases the effective pore diameter to 4.7 nm and broadens the pore size distribution by about eight times from the sharp distribution of the bare Gelsil sample. The superfluid properties were studied by a torsional oscillator for adsorbed film states and pressurized liquid states, both before and after the monolayer Kr preplating. In the film states, both the superfluid transition temperature $T_{\mathrm c}$ and the superfluid density decrease about 10 percent by Kr preplating. The suppression of film superfluidity is attributed to the quantum localization of $^4$He atoms by the randomness in the substrate potential, which is caused by the preplating--induced broadening of the pore size distribution. In the pressurized liquid states, the superfluid density $ρ_{\mathrm s}$ is found to increase by 10 percent by Kr preplating, whereas $T_{\mathrm c}$ is decreased by 2 percent at all pressures. The unexpected enhancement of $ρ_{\mathrm s}$ might indicate the existence of an unknown disorder effect for confined $^4$He.
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